The idea
A food chain is a single path: grass → grasshopper → frog → hawk. Reality is messier, because most organisms eat several things and are eaten by several others. Draw all of those arrows at once and you have a food web.
graph BT G["Grass"] --> GH["Grasshopper"] G --> MO["Mouse"] GH --> FR["Frog"] GH --> SP["Spider"] MO --> HA["Hawk"] FR --> SN["Snake"] SP --> FR SN --> HA HA --> DE["Decomposers"] SN --> DE FR --> DE
Arrows point in the direction the energy travels — from the eaten to the eater. Students get this backwards constantly; the arrow is not “eats”.
Reading a web
Two questions to ask of any food web:
- What happens if one species disappears? Trace every arrow leading out of it. Everything downstream is affected.
- Which species has the most arrows? Removing that one causes the most disruption. Ecologists call it a keystone species.
Sea otters and kelp
Otters eat sea urchins. Urchins eat kelp. Remove the otters and the urchin population explodes, the kelp forest is grazed to bare rock, and every species that lived in the kelp goes with it. The otter never touched the kelp.
Practise this in Reading a Food Web.
Curriculum connection
B2.2
explain how naturally occurring phenomena, including the cycling of matter and the flow of energy, contribute to the dynamic equilibrium within and between ecosystems
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B2.5
explain the effects of various human activities on the dynamic equilibrium of ecosystems
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